How to deal with extremely long front lines?

It's not the first time i got a very long border with some enemy. Right now it's South America cut sideways, but large, courved front lines in Asia also existed.

How do i cover so much ground, especially without risking my units too much?

If i use stacks of 10-15 units, there are not enough units.

If i use 1 scout car to land grab fast, they are very vulnerable particullary to air strikes by not even full stacks of attack planes of tactical bombers.

How do you deal with this problem?